This invention relates to the transfer of electrical energy from an energy source to a capacitive store and, more particularly, involves energy trapping and adaptive clocking of the energy transfer cycle in conjunction with a resonant circuit.
The charging of a capacitive energy store requires the transfer of energy from an energy source. Energy sources such as generators, batteries, fuel cells, and solar cells are typically voltage sources. The capacitive store initially appears as a short circuit when connected to a voltage source that has a voltage higher than that across the capacitive store. Consequently, the flow of current must be controlled.
The simplest control means is a series resistor as shown in FIG. 1. Voltage source 1 having a voltage Vdc charges capacitor 2, having a capacitance C, in series with resistor 3, having a resistance R. This circuit limits the peak current to a value of Vdc/R, and results in a relatively long charging time to achieve 99% Vdc, i. e., approximately 3RC seconds. The charging efficiency is only 50%; that is, resistor 3 dissipates the same amount of energy that is transferred to capacitor 2, or C/2Vdc2. In some low-energy applications, resistive charging is the best engineering choice. However, in high-energy applications, the relatively long charging time or the 50% efficiency is unacceptable.
The charging time and efficiency are improved by resonant charging. This accomplished by replacing resistor 3 with an inductor 4, having an inductance L, as shown in FIG. 2. The theoretical efficiency of resonant charging approaches 100% and is typically greater than 95% in practice. The charging time is given by xcfx80(LC)1/2 seconds, with the peak current being limited to V/(LC)1/2. Diode 5 is used in the circuit because the capacitor 6 charges to almost twice the voltage of d.c. voltage source 7, Vdc, and it is necessary to prevent the charge transferred to capacitor 6 from flowing back into voltage source 7.
The peak energy storage rating of inductor 4 is one-fourth the energy rating of capacitor 6. The specific energy of a capacitor is on the order of 2000 J/kg, and that of an inductor is typically much less, on the order of 50 J/kg. Therefore, inductor 4 is typically on the order of 40 times larger than capacitor 6.
In moderate low-energy applications, such as pulsed radar transmitters, resonant charging is a good engineering choice. However, when the capacitive stored energy is greater than a few kJ, a better alternative for the charging apparatus is a switching-type capacitive charging power conditioner, or xe2x80x9cSCCPC.xe2x80x9d The SCCPC operates from a d.c. source and provides fast and efficient charging of the capacitive store. In most applications, it also replaces the large d.c. power supply required for the input power by operating from a directly rectified a.c. power line. The SCCPC can also operate from any other suitable d.c. source, such as a battery.
A transformer is an important part of an SCCPC because it accommodates the difference between the voltage source and the load voltages, and isolates the voltage source from the load. Transformers must operate with bipolar voltages that contain no d.c. components. In general, transformers are inversely related in size and cost to the frequency of operation, which is motivation for operating the SCCPC at high frequency. There are two basic configurations of the SCCPC, the center-tapped transformer configuration shown in FIG. 3 and the xe2x80x9cHxe2x80x9d bridge switch configuration shown in FIG. 4.
The principle of operation is the same for both SCCPC configurations. A small amount of energy is measured out by the primary capacitor, switched through the transformer, then rectified and deposited into the load capacitor. This process is repeated at a high frequency until the load capacitor is fully charged and in a manner such that the transformer is subjected to only a bipolar voltage.
More particularly, center-tapped configuration 8 of the prior art is schematically illustrated in FIG. 3. Center-tapped configuration 8 operates by alternately charging small capacitors 9 and 10 by means of switches 11 and 12, from voltage source 13, through the primary winding of transformer 14. Transformer 14 usually steps up the voltage by a factor of N, i. e., N is typically greater than 1, where N is the turns ratio of a transformers secondary and primary windings; but in some cases the voltage may be stepped down, i. e., N may be less than 1. The secondary current of transformer 14 passes through bridge rectifier 15 and then into load capacitor 16. This process is repeated at a high frequency such that over a period, load capacitor 16 is charged to the desired voltage. The switches 11 and 12 are operated in an alternating sequence such that the voltage applied to transformer 14 is bipolar and has no d.c. component.
H-bridge circuit 17 of the prior art is schematically shown in FIG. 4. H-bridge circuit 17 has only one small primary capacitor 18, which is charged through the primary coil of transformer 19. The H-bridge switches 20, 21, 24 and 26 are sequentially operated to alternately apply a bipolar voltage through capacitor 18 to transformer 19. Specifically, in the first energy transfer cycle, the switch pair 20 and 26 are turned on, while switch pair 21 and 24 remain in the off state. This connects the positive side of voltage source 27 through small primary capacitor 18 to the top of the primary coil of transformer 19.
After this energy transfer cycle is completed, the next energy transfer cycle begins with switch pair 21 and 24 being turned on while switch pair 20 and 26 are switched to the off state. This connects the positive side of voltage source 27 through primary capacitor 18 to the bottom side of the primary coil of transformer 19, thus providing the reverse polarity and ensuring that the bipolar signal received by transformer 19 has no d.c. component. This sequence of operating two the switch pairs is repeated until the required amount of energy is transferred through bridge rectifier 28 to load capacitor 29.
The basic energy transfer process and the functions of the switches during a single switching event of the same polarity can be better explained using simplified equivalent circuit 30 shown in FIG. 5. Circuit 30 illustrates the operation of both center-tapped circuit 8 of FIG. 3 and H-bridge circuit 17 of FIG. 4.
Transformer 14 in circuit 8 of FIG. 3 and transformer 19 in circuit 17 of FIG. 4, are replaced in FIG. 5 by equivalent leakage inductor 31. The equivalent inductance of inductor 31 can be obtained by calculation familiar to those skilled in the electrical art, using the transformer turns ratio N. Likewise, load capacitor 16 in circuit 8 and load capacitor 29 in circuit 17 are represented by equivalent load capacitor 32. The capacitance of capacitor 32 can be calculated using equations and methods well known to those reasonably skilled in the electrical art. The voltage across load capacitor 32 divided by the voltage of source 33 is defined as the charge ratio xcex1. Forward switch 34 is a silicon controlled rectifier, or xe2x80x9cSCR,xe2x80x9d with parallel back diode 35. However, any suitable switch may be used, such as an isolated gate bipolar transistor, or xe2x80x9cIGBT,xe2x80x9d or monolithic oxide silicon field effect transistor, or xe2x80x9cMOSFET.xe2x80x9d
The operation of the switch cycle begins when forward switch 34 closes, i. e., is turned on. A resonant current flows from voltage source 33 through switch 34, through capacitor 36, through inductor 31, through the bridge rectifier formed by diodes 37, 38, 39 and 40, and into load capacitor 32. Being in a resonant circuit, the voltage across capacitor 36 will increase and ultimately exceed the voltage of the voltage source 33. When this occurs, forward switch 34 is turned off, and the current through capacitor 36 reverses and flows back through back diode 35, that is, across forward switch 34.
The reverse current deposits energy back into voltage source 33. This reverse current continues to provide a positive energy transfer to load capacitor 32 because the bridge rectifier allows only a positive flow of current into load capacitor 32, while at the same time routing the excess energy back to voltage source 33. As this reverse current continues to flow, it builds up an opposing voltage on capacitor 36 until the opposing voltage is sufficient to reduce the reverse current to zero. When the reverse current reaches zero, the energy transfer cycle is completed. Forward switch 34 is then turned on and the next energy transfer cycle is begun.
It can be shown that the energy transferred to load capacitor 32 is a function of the state of charge of load capacitor 32, and that the fractional transfer is very low when state of charge across load capacitor 32 is low. More particularly, the fractional amount of the energy transferred to capacitor 32 relative to the energy that is delivered to the circuit from voltage source 33, also known as the energy transfer ratio, is given by the following equation:                               J          ⁡                      (                          G              ,              α                        )                          =                  8          ⁢                                    (                              1                -                G                            )                        ⁡                          [                                                G                  ⁡                                      (                                          1                      -                                              2                        ⁢                        α                                                              )                                                  -                1                            ]                                ⁢                                    [                                                α                  ⁢                                      (                                          1                      -                      G                                        )                                                  -                2                            ]                                                                        (                                      1                    +                    G                                    )                                4                            ⁢              G                                                          (        1        )            
where G=the ratio of the capacitance of capacitor 32 to the capacitance of capacitor 36, typically 100 to 10,000.
During the initial stages of the charging process begins, the voltage on capacitor 32 is very low, and thus xcex1≅0. Under this condition, the energy transfer ratio J(G, xcex1) simplifies to the following expression:                               J          ⁡                      (            G            )                          =                              4            ⁢                          G              2                                                          (                              G                +                1                            )                        3                                              (        2        )            
Accordingly, the transfer ratio J(G, xcex1) is very low during the initial stages of the charging process, e. g., J(G, xcex1)≅0.004 for G=1000.
As the voltage builds up on capacitor 32, xcex1 increases, and thus the transfer ratio J(G, xcex1) also increases. Nonetheless, the average energy transfer ratio taken over the entire charging process is low, and this inefficiency requires a large number of cycles to achieve a useful energy transfer to capacitor 32. The energy not transferred to capacitor 32 from the energy delivered to circuit 30 during each cycle is returned to voltage source 33 by the reverse current.
Each energy transfer cycle is of a short duration and is repeated at a high frequency to accomplish the total energy transfer to load capacitor 32. The high frequency is a major factor in reducing the size of the transformer and thus the size and cost of the apparatus. However, the high frequency concomitantly imposes a high switching loss because the amount of energy that must be processed is much larger that the amount actually delivered to load capacitor 32.
The period of the energy transfer cycle, T, is also a function of the state of the charge ratio xcex1 of load capacitor 32. The following expressions approximate T for two conditions, T1 for xcex1xe2x89xa6⅔, i. e., during the initial cycles of the charging process, and T2 for xcex1 greater than  greater than ⅔, i. e., during the latter cycles of the charging process:                                                                         T                1                            =                              2                ⁢                π                ⁢                                                                            LC                      1                                        ⁢                                          G                                              1                        +                        G                                                                                                                                                                    when                ⁢                                  xe2x80x83                                ⁢                α                            ≤                              2                3                                                                        (        3        )                                                                                    T                2                            =                              π                ⁢                                                                            LC                      1                                        ⁢                                          G                                              1                        +                        G                                                                                                                                                                                                          when                    ⁢                                          xe2x80x83                                        ⁢                    α                                    ⟩                                ⟩                            ⁢                              2                3                                                                        (        4        )            
where:
L=the inductance of inductor 31; and
C=the capacitance of capacitor 36.
At the present time SCCPC""s are driven at a fixed frequency selected to accommodate the maximum charging cycle period that occurs at the beginning of the charging process, i. e., before the charge on load capacitor 32 has appreciably increased. As a result, the period is much longer than that necessary during the latter stages of the charging process, i. e., when xcex1 has significantly increased. Consequently, during a substantial portion of the total time necessary to complete the transfer of energy from voltage source 33 to load capacitor 32, i. e., during the latter stages of the charging process, approximately 50% of each charging cycle period is comprised of dead time, i. e., the period exceeds that which is necessary to drive the circuit.
It follows that there is a need in the art for a charging apparatus capable of transferring all of the energy taken from the voltage source in each switching cycle, while matching the clocking frequency to the period required for energy transfer for each cycle throughout the charging process.
A resonant switching-type capacitive charging power conditioner circuit includes a trap switch assembly to prevent the energy initially delivered to the circuit by an electrical energy source from returning to the source. Once trapped, all of the energy is transferred to a capacitive store, such as a load capacitor, over a number of cycles. The period for each cycle is a function of the state of charge of the capacitive store, and the period decreases for each successive cycle as the charge on the capacitive store increases to its final value. Switches are turned on and off in response to the absence of certain currents in the circuit, to match the decreasing periods of the successive charging cycles, respectively, throughout the charging process. This adaptive clocking prevents energy from returning the energy source, and eliminates dead time for each cycle.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention.